Distribution transformers have conventionally been protected from fault currents by load sensing fuses provided on the primary winding. An externally operable primary switch combined with a temperature sensing wax motor is disclosed in U.S. Pat. No. 4,053,938 issued on Oct. 11, 1977 entitled Temperature Sensing Transformer Primary Switch which is assigned to the same assignee as the present application. In this system, a wax motor senses the temperature of the insulating fluid and on reaching the melt temperature, opens the primary switch. The wax motor only sensed extended overloads, and could only interrupt load type currents, not fault currents. Load sensing fuses must be replaced on fusing before the transformer can be put back on line.